Various types of jacks and lifting devices are used in a wide variety of industries for lifting many different types of loads. Each of the various designs of jacks and lifting devices possesses distinct advantages and disadvantages. Thus, it is important to choose a design that is suited to the specific lifting requirements of the task at hand.
Scissor lifts, which are well known, are very compact and possess low height profiles. These characteristics allow scissor lifts to function when limited clearance is available beneath a load. Scissor lifts are also advantageous because they are simple in design, and provide a stable and parallel movement of the load-bearing surface with respect to the base of the lift. However, scissor lifts that are driven by horizontally-oriented actuators are very inefficient, particularly during the initial stage of the lift. During this initial stage, a relatively large amount of horizontal force is required to raise the load on the scissor lift because only a small percentage of the horizontal force is translated into vertical force.
Another well-known lifting apparatus is the wedge jack. Wedge jacks are advantageous in that they are capable of lifting very heavy loads and are also very simple in design. Shortcomings of wedge jacks include difficulty in designing a jack such that the load-bearing surface remains horizontally stationary as it moves vertically; issues with friction between the moving surfaces in the jack; and design constraints with respect to the positioning of the linear actuator that drives the jack as well as the overall housing of the jack, which must contain moving and stationary wedges over the range of the lift.
Both scissor lifts and wedge jacks share a common deficiency: if the linear force actuator supplying the force to lift the load fails in some way, the load is uncontrollably dropped to the resting position of the scissor lift or wedge jack. This can be particularly problematic when multiple scissor lifts or wedge jacks are being used since the load being lifted may become unbalanced and shift or slide unpredictably. The failure of such a lifting apparatus creates a risk of serious property damage as well as a safety concern for people working in the same area as the load. While some devices and techniques for preventing scissor lifts and wedge jacks from failing in this way are taught in the prior art, most of the safety measures disclosed are not integral to the design of lifting apparatus itself. These safety measures may fail of their own accord or be intentionally or unintentionally disengaged by an operator.
One particular situation that arises frequently and presents a number of challenges is where the load to be lifted is very heavy and there is minimal space in which to position a lifting apparatus underneath said load. A common example of this situation is where heavy vehicles such as construction machinery or military equipment must be lifted for maintenance or transportation purposes. Scissor lifts are not feasible for this purpose due primarily to their inefficiencies in the initial portion of the lift. Wedge jacks are generally too large and unstable for such a task, and are often incapable of lifting the load to the required height. Devices using vertically-oriented actuators are often used for such tasks, but these devices do not have compact, low height profiles and are often unable to fit between the load and the surface underneath. Such devices also carry the same safety risks as scissor lifts and wedge jacks with respect to the failure of the actuators used to lift the load.
It is, therefore, desirable to provide a hybrid wedge jack/scissor lift device that overcomes the shortcomings of the prior art lifting apparatus designs.